The long-term goal of this project is to use a sensitized genetic screen to isolate novel mutations affecting zebrafish embryogenesis. The ease of observation and manipulation makes the zebrafish embryo an ideal model system to study fundamental questions in vertebrate biology. Small- and large-scale screens have identified several dozen genes that when mutated affect embryogenesis. Molecular and phenotypic analyses of these genes have provided novel insights into developmental signaling pathways. Additional screens are necessary, however, since previous screens have not reached saturation and overlapping gene functions have precluded the isolation of genes that might play important roles during embryogenesis. We will combine largescale haploid screening with a sensitized genetic background. Previous double mutant analyses with zygotic one-eyed pinhead mutants have uncovered partially overlapping functions of genes acting during embryogenesis. This sensitizing property of one-eyed pinhead will be exploited by generating double and single mutants of one-eyed pinhead and chemically induced mutations. Modulating loci will be identified by detailed phenotypic analysis. This sensitized screen will provide important resources and fundamental insights into developmental signaling pathways in vertebrates.